This invention pertains to a servo means for correcting the output of a ring laser gyro by removing a properly scaled signal from its output signal to compensate for dithering of the gyro.
A frequent technique for using the ring laser is to superimpose a portion of the two counterpropagating beam modes to produce lines or bars that move across the field of an optical sensor. The optical sensor converts the interference lines or bars into electrical signals that are counted. Periodically, the count is latched, and that count is a measure of the incremental rotation angle sensed by the gyro.
When small angular velocities are measured, the beam frequencies of the two counterpropagating beam modes tend to lock together. To avoid mode frequency locking, it is customary to rock or dither the ring laser about its sensing axis. The dither motion between the ring laser and its mount is sensed, and it is subtracted from the total motion sensed by the ring laser gyro to obtain a measure of some external angular motion.
While the gyro is sensing angular motion, a pickoff, for example a magnetic or piezo-electric pickoff, senses the instantaneous angular position of the ring laser relative to the instrument mount. The sensed signal is converted into a digital signal, and it too is latched to get an instantaneous reading of the angular motion of the instrument relative to the instrument support. The difference between the signal from the ring laser and the signal from the pickoff is a measure of the angular motion of the instrument support about the sensor sensing axis relative to inertial space.
Errors are introduced by changing scale factors in the electronics and in the angular pickoff. Electrical, usually electronic, apparatus are used to modify the signals to compensate for such errors. Other approaches include optical detection which is not sensitive to dither motion. A more cost effective approach, which has been used successfully, consists of digital compensation of the gyro output signal using the system computer. The gyro dither pickoff signal is converted to a digital form which is easily manipulated by the computer. The digital signal is appropriately scaled and phase compensated, and it is then subtracted from the digital gyro counts to form a dither compensated signal. Servo loops in the computer track both the gain and phase of the pickoff signal to maintain their compensation. Unfortunately, when the gyro senses low frequency oscillatory angular motions or constant angular motions, a large signal is channeled through the feedback paths of the gain/phase tracking controller. The presence of such large or low frequency signals disturb the pickoff correction parameters, and it can cause the entire system to be unstable, failing to eliminate the interference of the gyro dither in the output signal of the gyro.